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Water and external systems

Site Drainage and Rainwater Management Systems

Water landing on a roof or a paved surface has to go somewhere, and it travels through a chain of systems to get there. Each entry in this family is one link in that chain, and the honest way to tell them apart is by what sits upstream and what sits downstream: a gutter hands to a downpipe, a downpipe hands to a gully or a soakaway, a swale conveys rather than stores, an attenuation system holds water back and releases it slowly.

Reading the handover points is what makes a drainage strategy legible, and it is also where most of the arguments on site happen. Waterproofing of buried structures sits with below-ground systems, and permeable surfaces sit with paving, because they are surfaces first and drainage second. Nothing here contains pipe or gully sizing, gradients, rainfall data, storage volumes, or any determination about connecting to a public sewer.

11 system entries · educational assemblies reference · how a system performs always depends on the whole building and qualified professional design

Scope

What this category covers

Where the boundary of this part of the encyclopedia sits, including what it deliberately leaves to qualified professionals.

  • Roof rainwater collection and the two different ways roof water is made to flow
  • Surface conveyance, infiltration, attenuation and treatment across a site
  • Subsoil and retained ground drainage, buried networks, and rainwater held back for reuse
  • Not covered: pipe and gully sizing, gradients, rainfall or storage figures, or connection approvals

All entries

Every drainage & rainwater entry

The complete set for this category. Frequently referenced entries come first — a starting point, not a ranking.

Roof Rainwater Drainage →The gravity arrangement of gutters, outlets, downpipes, overflows and discharge points that takes water off a roof and hands it on clear of the building base.Surface Water Collection →The site-scale arrangement of shaped surfaces, kerbs, gullies and linear channels deciding where water running across external ground is caught and where it is sent next.Buried Drainage Network →The buried pipe runs, bedding and surround, junctions, inspection access and connections that carry foul and surface water away from a building and its site to a sewer or another disposal point.Siphonic Roof Drainage →A roof drainage arrangement designed so that baffled outlets keep air out of pipework that primes and runs full once flow fills the bore, allowing horizontal collection and few vertical stacks.Vegetated Swale →A shallow vegetated open channel that conveys surface water slowly along a route while sediment settles and some water soaks away, sometimes with check structures and an underdrain.Land Drainage →A buried arrangement of perforated pipe within permeable fill, separated from the soil by a filter layer and laid out to intercept or lower the water held in the ground.Wall Base Drainage →The permeable path, filter separation, base collector and outlet points behind a retaining structure, arranged so that water does not accumulate against the wall and load it.Infiltration Drainage →A buried void or trench that receives collected water and gives it to the surrounding ground, protected by pre-treatment and a filter separation, with a defined route for what the ground refuses.Attenuation Storage →Storage set between a site's collection network and its discharge point, releasing water through a flow control so the rate leaving is limited, with a route for events beyond the assumption.Bioretention →A shallow planted depression built as a layered filter, where water ponds briefly, drains through an engineered medium and a transition layer, and then infiltrates or is collected below.Rainwater Harvesting →An arrangement that diverts roof water into storage, filters and holds it, and distributes it for non-drinking uses, kept separate from the mains supply and overflowing to the site drainage.

Elsewhere on Build Design Hub

Planning guidance for this subject

This encyclopedia explains how these assemblies are organised. These sections help with choosing between them and preparing for the conversation.

Drainage and site planning →The planning side of where site water is asked to go.Gutter and drainage planning →How roof water collection is approached in planning.Drainage survey preparation →What to gather before a drainage survey or discussion.

Building Systems Encyclopedia

Reference entries for building assemblies — how each is organised, what role every component plays, how the parts interact and which junctions matter.

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